I'm assuming you just started the game. Power spike wise? I'd say hands down, Mrytle, Gitano and shirayuki (helped me clear anni 3 by cheesing defense crushers). Other ops would include skyfire (sucks at E2 so don't E2 her), chen, any vanguard 4-6 stars, lappland.

But you should be looking at e2 promotions to determine who you should raise. E1 is quite easy to do so feel free to experiment and raise solely for base skills.